Cassini's Extended Mission, July 2008 to June 2010 Options

[john_s]

The winner is (drum roll please) PF6h9. Officially adopted on Thursday. I haven't sifted through all the details yet, but from my parochial point of view, I know it includes seven close Enceladus flybys, so that's good. Most of the science groups (Titan, Rings, Magnetosphere, Saturn, and Icy Satellites) were pretty happy with this choice- it packs in an amazing number of science opportunities.

John.

[volcanopele]

I'm not sure on all the details either. I'm not sure how the Enceladus flybys have changed, but good old PF6 had 2 encounters @ < 50 km, 2 encounters @ > 50 and < 200 km, 2 encounters @ > 200 and < 1000 km, and 1 encounter @ > 1000 and < 3000 km. There appear to be 26 Titan flybys with a good mix of S Polar, N Polar, and trailing hemisphere encounters.

[ugordan]

Grabbed from the Yahoo Cassini-Huygens group: Tour de Saturn Set For Extended Play

Excerpt:

+ Cassini will fly by Dione once at 500 km altitude: a moon which is long-dead now, but shows clear signs of violent tectonic stretching during the Solar System's ancient days -- and which also seems to be still releasing a tiny trickle of water vapor, only about 1/300 as much as Enceladus, from someplace on its surface.

+ The spacecraft will fly within 1500 km of Helene, one of Saturn's recently-discovered tiny moons that seem to be loose clumps of icy rubble stuck together only loosely by gravity. Helene, discovered in 1980, is one of two moons that share Dione's orbit at its gravitationally stable "Lagrange points". (Two more tiny Lagrange moons share Tethys' orbit.)

+ In addition, Cassini will skim the surface of Rhea, Saturn's second-biggest moon, at 100 km altitude. Unfortunately, Cassini's lack of a tiltable scan platform seriously limits its ability to cancel out the motion blur that fuzzes up its images during its really close flybys of various moons.

[john_s]

Here's a file listing the geometries of the icy satellite flybys in the chosen extended mission tour. Close encounter geometries may change slightly as the tour is fine-tuned, but by no more than a couple of thousand kilometers, and the dates will not change.

John
 Icysat_Flybys_UTC_NFB.PF_6h9L.txt ( 14.77K ) Number of downloads: 1427

[djellison]

My word - 17km/sec to an altitiude of 25km...that's FAST. What are the typical exposure times for ISS NA and WA using clear filters?

We're talking a 150 metre footprint for the NA - so 14.6 cm/pixel - but wow - an exposure of, say, 1 second - would blur the image by 116,000 pixels Take a zero off the end for the WA

Doug

[NMRguy]

Looks like we should get a better look a Mimas on 13 Feb 2010. Phase angle of 99 and distance of about 10,000km. Although it still isn't a "close flyby", it will be about 6.5 times closer than the previous best. And frankly, I wouldn't mind getting a little sharper look at that heavily cratered surface.

[ugordan]

What are the typical exposure times for ISS NA and WA using clear filters?

I see 50-ish ms for NA as typical for Enceladus at phases around 30 degrees. This is in 1x1 binning mode, a 2x2 binning is more than likely to be used due to motion blur so you can get something like 15 ms for NAC. Or 5 ms for 1x1 WAC.

[john_s]

The super-close flybys will be mostly for fields and particles (or gravity), not remote sensing- in fact we may try to increase the range of some of the flybys to make them more suitable for remote sensing.

[djellison]

5ms @ 17km/sec - 85 metres.

2 x 2 WAC - 3m/pixel - still HUUGEEEE ammounts of blur - BUT - on the way in and the way out again, NA should be able to get some spectacular images.

Doug

[volcanopele]

sweet! Thanks, John.

Yeah, don't expect super hi-res images from those very close flybys...

In terms of Enceladus flybys, I am disappointed by the apparent lack of north polar looks during the latter part of the extended mission. Nothing too close would be needed, but it would be nice to fill out the global map at sufficient resolution. There are quite a few close passes over the south pole, though, 2 flybys at around 100 km altitude, and another at 1811 km, though there will likely be a push to see if that one can be lowered, me thinks. All three of those are at decent flyby speeds @ around 7 km/sec. so good data should be obtained from them.

[john_s]

I just did a blog for the Planetary Society about the extended mission tour selection process- here it is

John.

[djellison]

Was just about to post a link to it - excellent piece John - Suprised to see that Dave doesn't actually have a variety of orbits visualized above his head in some sort of orbital mechanics halo But then - if you put your OSX dock on the right side...there's just wierd...

Doug

[jamescanvin]

But then - if you put your OSX dock on the right side...there's just wierd...

That's not weird, just different! I have mine on the right - makes much more sense to have it there on a widescreen display.


Great article John.

James

[djellison]

Well - when i actually get a Mac (this summer) then I'll speak with confidence...

Actually - it does kind of make sense

Doug

[Bjorn Jonsson]

Looks like a great extended mission. Big thanks to John for posting the geometries of the icy satellite flybys - very useful. The blog entry was also interesting. Nice to see Mimas getting a close flyby, meaning that over the entire mission each of the icy satellite is getting at least one close flyby. This is far better than expected some years ago. Very close flybys of Phoebe and Tethys weren't expected back then.

Now I hope some SPICE kernels for the extended mission get released soon so I can get a detailed look at the entire 2 year mission extension. In particular I wonder if Saturn's northern hemisphere is going to get globally imaged like the southern hemisphere in September and October 2004 (if so, probably from closer range and over a shorter time though) so I can complement my 25 degrees/pixel cylindrical map of the southern hemisphere with a comparable map of the northern hemisphere .

One interesting tidbit I noticed in the FY2008 budget documents released today: "Cassini was launched in October 1997, arrived at Saturn in July 2004, and will continue to investigate Saturn and its major moon through September 2012."

[Guest_Sunspot_*]

Well - when i actually get a Mac (this summer) then I'll speak with confidence...

Actually - it does kind of make sense

Doug

I'm getting a Mac Mini when Leopard is released. I've been using Windows ME since January 2001.

[Decepticon]

NT encounters will still happen? I don't mind Some of these far off looks!

[volcanopele]

Great blog post, John_s!

I noticed something in the image used that I didn't know. The last item listed for each tour is the Delta-V used in each tour. From a Titan perspective, the lower that number the better, as a lower delta-v would allow for a longer, extended-extended mission. The tour selected, PF6h9, has the highest delta-v of the tours in that Excel chart. IIRC, at the end of the nominal tour, there should be around 300 m/s of delta-v left...

[ugordan]

That certainly puts into perspective what we can expect from an extended-extended mission. These figures might be conservative delta-V costs, though.

[TritonAntares]

As presumed and the list above displays Iapetus will not be visited closely by CASSINI during this Extended Mission,
but what about far-encounters in a few 100.000 kilometres?
Remember the second closest approach to this strange moon so far on new year 2005 was in roughly 120.000 km!
There will probably be some occasions like this again, but only showing the saturn facing hemisphere with the dark Cassini Regio.
Due to Iapetus 15° inclination such far encounters could give the opportunity to have an additional look at northern or southern polar regions as well.

Bye.

[EccentricAnomaly]

The tour selected, PF6h9, has the highest delta-v of the tours in that Excel chart. IIRC, at the end of the nominal tour, there should be around 300 m/s of delta-v left...

The DV for the extended mission will probably total around 230 m/s when all of the tweaking is done and maneuvers that fall during good science are moved. The tanks are estimated to have about 340 m/s are the start of the XM, so that leaves only ~100 m/s for the XXM and spacecraft disposal (either crashing or putting into a stable parking orbit). But back when the tour was changed for the Huygens mission we thought we would only have 130 m/s for the XM, so 100 m/s for the XXM isn't that bad when put into perspective.

In many ways the XM is more intense than the prime misson... it will be very exciting.

[volcanopele]

As presumed and the list above displays Iapetus will not be visited closely by CASSINI during this Extended Mission,
but what about far-encounters in a few 100.000 kilometres?

I'll take a look.

EDIT: Here are the closest I could find:

2009 07 05 05:00:00 - 2,089,800 km (low phase, sub-saturn hemisphere, eastern Cassini regio, Snowman craters)
2009 09 11 19:47:00 - 1,161,000 km (high-phase leading hemisphere, extreme eastern Cassini Regio, western snowman crater)
2009 12 01 15:07:00 - 1,377,900 km (moderate phase (~100 deg.) , southern leading hemisphere, extreme eastern Cassini Regio, western Snowman crater,western Terra Incognita)
2010 02 21 10:00:00 - 1,469,800 km (moderate phase (~50 deg.), southern sub-saturn hemisphere, view of eastern Cassini Regio, Snowman craters, Terra Incognita)
2010 05 08 16:30:00 - 1,234,800 km (high phase (~110 deg.), southern leading hemisphere, crescent view of extreme eastern Cassini Regio and western Terra Incognita)

That appears to be about it. Sorry, TA! Hope the Sept. 2007 encounter is a REALLY good one!

EDIT to the EDIT: terrain coverage descriptions now accurate

[john_s]

Here's a list of Titan flybys in the PF6h9 tour:
 Titan_Flybys_UTC_NFB.PF_6h9L.txt ( 11.58K ) Number of downloads: 1324


And here's a list of small satellite flybys:
 Rocks_Flybys_UTC_NFB.PF_6h9L.txt ( 30.06K ) Number of downloads: 1166


Titan flyby geometries won't change much in the fine-tuning process, because you can't change Titan flybys without changing the rest of the tour. I'm not sure how stable the "rock" flybys are, though.

John

[JRehling]

Here's a list of Titan flybys in the PF6h9 tour:
 Titan_Flybys_UTC_NFB.PF_6h9L.txt ( 11.58K ) Number of downloads: 1324

Wow -- 27 flybys. A few of them are a little farther than normal, but that's still a large number -- 60% of the main mission's flybys. I'm eager to see the plans for RADAR coverage and at least one good look at the northern latitudes post-equinox. (Actually, at equinox, it will take a few looks to see all of the north polar region.)

[elakdawalla]

Just for grins, I took John's spreadsheet of the small moon encounters, plugged in the moons' diameters, calculated the pixel size at closest approach, and figured out how many NAC pixels across the moonlets would theoretically appear if they were, in fact, imaged at closest approach. Poor little 3-kilometer Methone -- nine of these relatively close flybys, and the biggest it'll appear will be 21 pixels across! The story isn't much better for 4-kilometer Pallene. But most of these moonlets will be quite well imaged by the end of the extended tour. I can't wait to put together a family portrait after these flybys are done.

Here's the best of each (and do remember that these are theoretical maxima; the actual number depends on where in the flyby Cassini snaps the pictures; and also keep in mind John's caveat that the flyby altitudes can change a lot with itty bitty tweaks to Titan flyby altitudes):
Atlas: 189 NAC pixels
Calypso: 145
Epimetheus: 856
Helene: 3,512
Janus: 995
Methone: 21
Pallene: 24
Pan: 68
Pandora: 608
Polydeuces: 66
Prometheus: 670
Telesto: 209

--Emily

Attached File(s)

 JS_Rocks_Flybys_UTC_NFB.PF_6h9L.NACpixels.txt ( 6.87K ) Number of downloads: 363

 

[TritonAntares]

2009 07 05 05:00:00 - 2,089,800 km
2009 09 11 19:47:00 - 1,161,000 km
2009 12 01 15:07:00 - 1,377,900 km
2010 02 21 10:00:00 - 1,469,800 km
2010 05 08 16:30:00 - 1,234,800 km
That appears to be about it. Sorry, TA!
Hope the Sept. 2007 encounter is a REALLY good one!

BAD NEWS...

EDIT to the EDIT:
please ignore descriptions of what terrain is visible.
Viewing software has incorrect rotation rate for moons.

Uuuu....
Only incorrect rotation rates or more?

[volcanopele]

Distances are correct, phase angle is correct, times are correct. However, rotation rate, and thus the terrain visible, is wrong.

EDIT: Terrain descriptions in above post now edited and accurate.

[volcanopele]

while that highest possible resolution for Helene would be nice, I'm not sure how much we'll get, probably a couple of snapshots, no time for a nice mosaic. The flybys is high-phase incoming, and low-phase outgoing. Helene will become smaller than the ISS NAC field of view at around 5 minutes after C/A.

[elakdawalla]

Now that an extended mission tour has been selected I thought it'd be a good idea to open a new topic for the discussion. I'm moving some of the posts from the "Primary Target for Extended Mission" poll into this topic.

--Emily

[EccentricAnomaly]

Poor little 3-kilometer Methone -- nine of these relatively close flybys, and the biggest it'll appear will be 21 pixels across!

I think the reason why there were no targeted rock flybys in the prime mission was that no one knew the ephermerides well enough to target these bodys. I don't know how well the rocks are known now, but I'd bet Methone, Pallene, and Polydeuces aren't known very well... being recently discovered and all would mean there would be a lot less data available to calculate their orbits.

[TritonAntares]

EDIT: Terrain descriptions in above post now edited and accurate.

Many thanks, seems like we'll get the Iapetus view we're used to until now.
Maybe some southern polar regions and the Terra Incognita will be added,
if they haven't already been mapped during this september's fly-by.
I'm afraid we won't get a close up view of the Snowman at all...
Then saturnshine images from 2004-12-31 in ~120.000 km distance will still be the best.

Bye.

[ugordan]

All is not lost, TritonAntares. Remember we might still get an extended-extended mission which will necessarily be less flyby intensive than this one. Who knows what kind of orbits we'll be running then, if they decide to stretch the remaining fuel out it'll probably imply extended orbits. We might get apoapses pretty far out and just maybe get a nice nontargeted flyby of Iapetus. It's up to Iapetus to prove worthy of another look after we see the results of the September flyby.

Personally, though, I wouldn't hold my breath for new mysteries.

[JRehling]

Then saturnshine images from 2004-12-31 in ~120.000 km distance will still be the best.

Bye.

Of course, it's a given that all saturnshine images will cover the same hemisphere, since Iapetus keeps the same face pointed towards its primary. Unless a close-up in saturnshine comes in the XXM, new saturnshine images are going to add almost nothing to what we've already seen.

In fact, we can be sure that most any nontargeted imagery will show us more of what we've seen, because Iapetus is so far out that it will always be outside of Cassini's apoapsis unless Cassini's really making an effort to get to Iapetus. We will basically see the saturnside view unless a deliberate effort is made to do otherwise.

[TritonAntares]

...
In fact, we can be sure that most any nontargeted imagery will show us more of what we've seen, because Iapetus is so far out that it will always be outside of Cassini's apoapsis unless Cassini's really making an effort to get to Iapetus. We will basically see the saturnside view unless a deliberate effort is made to do otherwise.

I'm aware of this recuring viewing situation, only minor changes are possible due to CASSINI's position to Iapetus on its 15° tilted orbit.
I hoped that - apart from the Sep.10th fly-by - there would have been some opportunities to catch the moon from a few 100.000 kilometres like before under different illumination conditions and/or showing higher latitudes.

Bye.

[Holder of the Tw...]

I'm guessing we're going to have to wait until NASA officially endorses the extension before Solar System Simulator or Saturn Viewer show anything beyond July 2008 for Cassini.

[Guest_Analyst_*]

Does someone know the deterministic part of the delta v needed? I assume the statistical part (mean of zero) is not a small fraction (2 or 3 sigma) and good navigation can reduce this during the tour and give us more delta v remaining after the extended mission. This is why we now have about 340 m/s (any proof for this number?) after the prime mission and not much less as per plan from 2004.

Btw., has someone a current paper/document about the propellent usage/delta v? I know there is a site at JPL but it is restricted. Kind of comparsion of predicted vs. actual.

Analyst

[Guest_AlexBlackwell_*]

Btw., has someone a current paper/document about the propellent usage/delta v? I know there is a site at JPL but it is restricted. Kind of comparsion of predicted vs. actual.

As you might guess, the Cassini Program doesn't really like to make that type of information generally (i.e., easily) available. Just ask the poster of these two posts.

[tasp]

If I understand the September flyby geometry correctly, we should be getting some nice views of the 'equatorial dots' on Iapetus. (IIRC, these are also just visible in the Voyager 2 images, also)

Is it possible in this time frame to anticipate the detailed appearance of the 'dots'?

Perhaps.

Should a gaseous, thermoreactive material be resonsible for the darkening of Cassini Regio, and further, that the reactive materials being preferentially introduced to the Iapetan environment during passages through the Saturnian magnetotail, then the gradation between the lower, dark Cassini Regio colored areas and the upper (presumably higher altitude) light colored areas should have distinctive characteristics.

For instance, the width of the area between full saturation of the dark coloring, and the uncolored, white areas will be strongly slope dependent. A shallow slope producing a relatively wider transiton between dark and light, and steeper slopes producing a relatively narrower band between dark and light.

Additionally, should the slopes be deeply incised, we will see corresponding incursions of the dark staining into the 'ravines'.


Should the peaks be approximately cone shaped, the banding will be surprisingly regular about the peak, and dramatically uniform in appearance.


If Cassini images of these features show the 'dots' to be stained accordingly, we might be pretty confident in our understanding of the Cassini Regio dark staining, and most probably, the dark bottomed craters of Hyperion, too.

[Guest_Analyst_*]

Just ask the poster of these two posts.

These two posts are yours.

[tasp]

Additionally, craters on the slopes (should there be any) will modify the local slope angles, and the subsequent darkening will be modified accordingly. As in the upper latitiudes of Iapetus, I suspect this trait of the darkening will be more pronounced in the lighter (higher elevation) areas.

We might see a concensus form that the dust from Phoebe idea just doesn't explain Cassini Regio . . .

[JRehling]

Additionally, craters on the slopes (should there be any) will modify the local slope angles, and the subsequent darkening will be modified accordingly. As in the upper latitiudes of Iapetus, I suspect this trait of the darkening will be more pronounced in the lighter (higher elevation) areas.

We might see a concensus form that the dust from Phoebe idea just doesn't explain Cassini Regio . . .

I'll go out on a limb and opine that the dark stuff will not appear at all on the white peaks, and the stratigraphy will reveal that the darkening ended before the creation of the peaks (ergo, a very long time ago).

[tasp]

Some kind of eruption onto a darkened surface would be very different.

And hopefully we get pictures that are good enough to distinguish between the two.

On going eruptions coupled with on going darkening would be interesting. But I will say the peaks are ancient, and the dark coating is replenished either continuously, or renewed on timescales short compared to the age of the solar system.

[tasp]

How many different scenarios can we come up with that create white peaks/dark surroundings that might be discernably different to the Cassini lens?

[Guest_Analyst_*]

I have done a limited analysis of the propellant usage so far.

Sources are:
[1] Cassini Mission Plan
[2] Cassini Significant Event Reports
[3] Propellant Remaining per October 2005 given by AlexBlackwell
[4] Cassini Tour Redesign for the Huygens Mission
[5] Initial Cassini Propulsion System In-Flight Characterization
[6] Cassini Maneuver Experience: Launch and Early Cruise
[7] Cassini Maneuver Experience: Finishing Inner Cruise

The description of the txt file follows:

In head are four blocks, from left to right:

- specific impulse and thrust of the main engine from source [5], the thrust I used is slighly lower than the nominal 445 N; there are several reportings of a small engine underperformance (less than 1%, resulting in slighly longer burn times), 441 N helps to match the numbers, but the errrors using 445 N is very small too
- masses from source [5]
- propellant used until specific dates from sources [3] and [5]; these numbers are a little iffy: in source [5] the numbers are given for EOY 2001, not 01.04.2002; in source [3] for “late October”, these data are used only to guess the monopropellant usage
- monopropellant remaining as per specific dates; the number for 30.06.2004 (SOI) is computed as is the usage per day for cruise (about 6 g/d) and tour (27 g/d)

The 11 columns in the table are:

(1) TCM or OTM number
(2) the maneuver date, can be off be one day because times were given in UTC and PST and I didn’t care to check
(3) maneuver name/event
(4) maneuver duration; for TCM 1 to 17 (including) this has been computed using the rocket equation and the delta v given in column (5), for all other maneuvers these are actual numbers from source [2]
(5) actual delta v using biprop main engine; source [5] for TCM 1 to 17 (including), source [2] for TCM 18 and later
(6) actual delta v using monoprop thrusters; sources like column (5)
(7) and (8) predicted delta v for the tour from source [4]
(9) the computed delta v using the rocket equation, the given maneuver duration (4) and the propellant usage from coloum (13)
(10) the difference between (5) and (9), is of course zero for TCM 1 to 17 (including)
(11) the remaining monoprop using the numbers from above (6 or 27 g/d)
(12) the biprop remaining before the maneuver
(13) the biprop used during the maneuver computed using the duration (4), isp and thrust

I only care about biprop, the monoprop usage is assumed to be linar and monoprop delta v maneuvers are discounted. The monoprop tour delta v has only been 3.4 m/s so far.

The numbers match very good. For instance the computed DSM duration is off by about 20 s (less than half of a percent), the computed bipropellant remaing in late October 2005 is 493 kg vs. 499 kg given by [3] (about one percent error). The delta v difference for SOI is a little large (5 m/s, still less than one percent), I don’t have the exact burn time and used 96 minutes.

The biprop delta v after OTM 92 with 437 kg is 540 m/s, the mean of the remaining prime tour is 202 m/s ([4], old source), coming nicely to the 340 m/s after the prime tour given in this thread.

Any suggestions? I have a .xls file if someone cares.

Analyst

Attached File(s)

 Cassini.txt ( 23.27K ) Number of downloads: 463

 

[Rob Pinnegar]

I'll go out on a limb and opine that the dark stuff will not appear at all on the white peaks, and the stratigraphy will reveal that the darkening ended before the creation of the peaks (ergo, a very long time ago).

That would be a wild result if it turned out to be true. It would be very difficult to explain the lack of bright craters in Cassini Regio if the dark material were that old.

[tasp]

So, we seem to be at the point of saying dark dust deposited on Cassini Regio from Phoebe will look a certain way.

And eruptions of light colored materials onto a dark surface will look a certain way.

And a gaseous, thermoreactive darkening substance, introduced into the Iapetan environment during passage through the Saturnian magnetotail and either totally dissipated or totally used up in much less than the ~80 day period of Iapetus will look in a certain way.


And Cassini orbiter will get us the definitive pictures in September . . . .

[EccentricAnomaly]

I have done a limited analysis of the propellant usage so far.

Sources are:

[...]

Any suggestions? I have a .xls file if someone cares.

Analyst

I've seen papers from the Cassini maneuver team at AIAA conference that give great detail of the DV used for maneuvers. You can find them at aiaa.org by searching for the keywords 'Cassini Maneuver' in their paper database.

[Guest_Analyst_*]

Thank you. I have some of these papers until about 2002. These were (or still are?) available for free. But papers at aiaa.org cost $ 25.00 to 30.00 each. At such prices my monetary interest is tops my scientific one.

Analyst

[belleraphon1]

I agree that, moonwise, Titan and Enceladus are the prime targets for extended mission exploration....

But....

For those who say "one more flyby will tell us all we need to know about Iapetus". Reminds me of all those squggly line scientists who once said "Viking gave us all the pictures of Mars we will ever need".

Let's wait for this years Iapetus close flyby and I am sure more mysteries will unfold. Nature is never as nice and neat as our paper calculations........... and those equatorial mountains are saying something very significant about this world's (and the Saturn system) development.

I hope that another Iapetus flyby can be fit into the second extended mission. Otherwise we wait for the next dedicated Saturn orbiter (which will undoubtably focus on Titan or Enceladus) that can spare some time to scan that mountain range....... and at my age I will probably be viewing that from some assisted living facility (yuck..... rather have the ice at Headlands on Lake Erie take me).

Craig

[ugordan]

Reminds me of all those squggly line scientists who once said "Viking gave us all the pictures of Mars we will ever need".

Yes, but Mars was/is geologically active. Iapetus looks like it's... well, dead. Probably been for a very long time now. The question is if it even differentiated, it sure as heck has one lumpy shape with all those enormous impact basins.

I like to think about Iapetus the same way as Phoebe. How much more would we learn if we had another close flyby? I'm guessing not that much. Don't get me wrong, Iapetus is one of my favorite Saturnian moons from an imaging point of view and I'd like to see more close-ups just as the next guy. Scientifically, I don't know how interesting that would be.

[EccentricAnomaly]

Thank you. I have some of these papers until about 2002. These were (or still are?) available for free. But papers at aiaa.org cost $ 25.00 to 30.00 each. At such prices my monetary interest is tops my scientific one.

Analyst

yeah it bugs me that AIAA charges so much (even when they don't hold the copyright the expect you to pay them for photocopies). The best I can do is to recommend a nearby university library... often computers in the library can download these papers.... I got one paper this way... and I'd attach it, but it's too big (1.7 MB)

[belleraphon1]

Yes, but Mars was/is geologically active. Iapetus looks like it's... well, dead. Probably been for a very long time now. The question is if it even differentiated, it sure as heck has one lumpy shape with all those enormous impact basins.

I like to think about Iapetus the same way as Phoebe. How much more would we learn if we had another close flyby? I'm guessing not that much. Don't get me wrong, Iapetus is one of my favorite Saturnian moons from an imaging point of view and I'd like to see more close-ups just as the next guy. Scientifically, I don't know how interesting that would be.

Of course I want to concentrate on Titan and Enceladus.....

BUT..................AHHHHHHHHhh... but even the Dead Tell Tales.... (and no, I do not watch CSI). Something very interesting happened(s) at Iapetus that affected the outer reaches of the Saturn system at least inwards to Titan. And the Jupiter system has taught us that "old" surfaces can hide young interiors.... look at the hints of oceans under Caliisto's battered exterior... a moon that is not even completely differentiated. And Ganymede, cratered as he is, has a significant magnetic field and is completey differentiated.

Do not just write off what stories Iapetus has to tell.... I am always reminded of Poul Anderson's ditcum that every body is a " world" with a unique history and it's own personality.

Another reason I want to see us return to the Moon... that "dead" world will teach how to really respect the dead. And live under very harsh conditions.

Craig

[JRehling]

That would be a wild result if it turned out to be true. It would be very difficult to explain the lack of bright craters in Cassini Regio if the dark material were that old.

True. Pretty much destroys my assertion, in fact.

There are small craters with bright rims but dark floors that show us that the dark stuff was emplaced after those craters were formed, and this pretty much tells us that the dark stuff happened in the last fraction of Iapetus's history. I should have reviewed this before posting.

[JRehling]

So, we seem to be at the point of saying dark dust deposited on Cassini Regio from Phoebe will look a certain way.

And eruptions of light colored materials onto a dark surface will look a certain way.

And a gaseous, thermoreactive darkening substance, introduced into the Iapetan environment during passage through the Saturnian magnetotail and either totally dissipated or totally used up in much less than the ~80 day period of Iapetus will look in a certain way.
And Cassini orbiter will get us the definitive pictures in September . . . .

I think there's a pretty good chance of that.

They can take a stab wound and determine where the assailant was relative to the victim. Or a bloodstain and determine the geometry of a gory deed.

The interaction between topography and the dark-light pattern is going to tell us a lot we don't know already.

We already know a lot, and it seems to thwart a lot of simple models. Particularly, the Snowman craters give us some trouble. But we've mainly seen the eastern border of Cassini Regio. The September flyby will give us a great look at the western border. That alone will tell us if CR was emplaced from east to west, west to east, or from inside out. Or none of the above, also a real possibillity.

I'm reminded of the pre-Apollo situation with three theories about the origin of the Moon. It turned out that a combination of two of them best explained the data. Maybe Cassini Regio had a single catastrophic event as its origin, but it has had a continuing evolution since then.

Callisto may offer a useful analogue in terms of the interaction of dark surface with brighter, more volatile ice. Iapetus must be a little more complex than that, but that may partly explain things.

[dvandorn]

There are small craters with bright rims but dark floors that show us that the dark stuff was emplaced after those craters were formed, and this pretty much tells us that the dark stuff happened in the last fraction of Iapetus's history. I should have reviewed this before posting.

That's one interpretation. Another interpretation of the craters with bright rims and dark floors is that the entire crater was once all dark, but mass wasting has exposed brighter materials in the rims.

I tend to agree, though, that the dark stuff probably was emplaced over top of the craters, and happened pretty much after all of the visible craters were formed.

-the other Doug

[edstrick]

Iapetus is so battered that extra-high resolution will have rather little information to yield on global geology, at least directly, but global coverage is needed at good resolution and sun angles just to see the global geology well.

Where resolution higher than Cassini can provide (except for perhaps one tiny spot) is infrared spectral mapping for composition. Looking at steep crater walls and other topographic features to see the intimate mixing and distribution of ices and the darker materials will give a better understanding of their stratigraphic relations and transport processes and thus the geologic history of the albedo patterns and ice migrations.

Cassini won't be able to do much of that, unfortunately, even on it's fall flyby.

[TritonAntares]

...
The interaction between topography and the dark-light pattern is going to tell us a lot we don't know already.
We already know a lot, and it seems to thwart a lot of simple models. Particularly, the Snowman craters give us some trouble.
But we've mainly seen the eastern border of Cassini Regio. The September flyby will give us a great look at the western border.
That alone will tell us if CR was emplaced from east to west, west to east, or from inside out. Or none of the above, also a real possibillity.

Iapetus is so battered that extra-high resolution will have rather little information to yield on global geology,
at least directly, but global coverage is needed at good resolution and sun angles just to see the global geology well.

The basic problem so far has been the lack of encounters in mid-range distances - as far as I remember
one in 120.000 km, 600.000 km and 800.000 km - to poor to allow a satisfiable resolved mapping.
Due to CASSINI's orbit unfortunately only the saturn facing hemisphere could be observed so far.
Not comparable to all other saturnian moons - even the Phoebe encounter showed more...
Septembers close outbound encounter will improve this situation a lot for sure, but still enough regions will only be bad imaged - for instance the northern polar regions (Voyager images being still the best) or the Snowman with its dark terrains.
Hopefully an extension of the extended mission will give us the opportunity to visit Iapetus once more in an outbound passage over this regions not exactly so close as this fall.
If feasible an elongated orbit with its apoapsis outside of Iapetus' course - and eventually a bit tilted - would be favorable.
So Iapetus could be passed several times in say a few 10.000 to 100.000 kilometres showing different longitudes under changing illumination conditions.
But this is certainly at first a propellant question I guess...

Bye.

[JTN]

Have more details of the extended tour been made public yet?

At the moment, I'm particularly interested in what will happen around the solar ring plane crossing in August 2009. What sort of inclination will Cassini have at this point? Are there any specific opportunities to see the shadows of satellites stretching across the rings? (I realise the latter doesn't necessarily require knowledge of the tour.)

john_s mentioned the RPX in his glog entry, but without much detail of what Cassini will be up to.

I wonder whether there will be interesting deductions to be made from the thermal properties of the ring particles as the sunlit and dark sides of the rings swap places -- we will presumably be able to look at both sides more-or-less simultaneously for the first time.

[pat]

Have more details of the extended tour been made public yet?

At the moment, I'm particularly interested in what will happen around the solar ring plane crossing in August 2009. What sort of inclination will Cassini have at this point? Are there any specific opportunities to see the shadows of satellites stretching across the rings? (I realise the latter doesn't necessarily require knowledge of the tour.)

during the equinox period the inclination profile is something like:

~45 deg up to DOY 205
~35 deg DOY 205 to DOY 221
~21 deg DOY 221 to ~DOY 237
~12 deg from DOY 237

the process for deciding what observations Cassini will make during extended mission has only just begun. At the moment preparations are being made for the division of the extended mission into periods of time and allocating those periods to TWTs (Target Working Teams) and OSTs (Orbiter Science Teams). The process is called "segmentation" and the periods of time "segments". Segmentation will be completed by sometime this summer.

The TWTs and OSTs will then decide what observations will be carried out in their segments in a process called "integration", which can obviously only start after segmentation has been completed.
Integration may take a year or so to complete, I expect the extended mission will be intergrated from the beginning forwards as its possible that the XM might actually begin (July 2008) before integration has been completed.

Right now Cassini science team members will have a good idea of the type of observations they want to make and when they can make them but simply won't know until a segment has been integrated the exact observations and times. Its worth mentioning that there will be far more requests for observation time than there is time in the XM and only a fraction of requests will actually get allocated time and be included in the SOP(Science Operations Plan -- I think) during integration.

The TWTs and OSTs are made up of people from the Cassini science teams, any science team member who is interested in that TWT/OSTs general area of responsibility can participate, supported by Science Planning Engineers from the Cassini Project at JPL. This means that it is the Cassini scientists themselves who decide what science Cassini will perform, of course there are various safety constraints and engineering requirements where the Cassini Project at JPL decides but by and large its the scientists themselves who determine what Cassini does.

[Gsnorgathon]

I'm less interested in the shadows of the moons upon the rings as I am in the shadows of the rings upon themselves. I expect that will be quite a sight.

[JRehling]

I'm less interested in the shadows of the moons upon the rings as I am in the shadows of the rings upon themselves. I expect that will be quite a sight.

Good point. Something you can notice on a brick wall when the sun is almost coplanar with it: The first/last shadows become incredibly stretched out as 1/sin(0) goes to infinity and the shadow becomes a kind of infinitely powerful form of magnification of the wall's topography. The right image at the right time could provide what is essentially the highest magnification "image" in the history of space exploration (well, not counting ones taken within 1 km [cm] of the object in question).

[Rob Pinnegar]

It should at the very least give us a good idea of the thickness and vertical density profile of the F Ring, and perhaps some of the other rings too.

[Guest_Analyst_*]

I checked my calculated numbers of propellant remaining posted in this thread against some numbers given in this document from March 28th 2006 (page 3).

For March 28th 2006 I calculate a spacecraft mass of 2.706 kg, the document gives 2.710 kg. DSM, SOI and PRM used 2.086 kg calculated vs. 2.083 kg given. So I conclude my calculations are pretty much correct.

Based on this we have right now a spacecraft mass of 2.649 kg including 433 kg biprop and 91 kg monoprop. This translates into a biprop delta v capability of about 529 m/s.

Analyst

PS: The propellant remaining number given in the above document (422 kg) seems to be wrong.

[Greg Hullender]

Impresssive piece of analysis! You chose your handle well. :-)

One thing that confused me at first: in America a mass of "2.649 kg" would just be 2649 grams -- not much at all! Confused me for a bit, until I realized what was happening.

Question (for everyone): Is this business of swapping comma and decimal point a US/Europe thing, or an English/non-English thing? That is, for the speed of light (for example), who writes 299.792,458 km/s vs. 299,792.458 km/sec?

--Greg

[helvick]

It's country specific. South Africa uses "," for decimal position and "." as a thousands separator. The Irish and British definitely use "." and "," in the same way as the US. The French use a space for thousands separation and the Swiss use an apostrophe. It gets even more different if you move outside of the western hemisphere - you cannot assume that number grouping always happens in threes for example (Japan and China group numbers using a space as a "10 thousands" separator and a "." as a decimal).

Quite mad.

[dvandorn]

Quite mad.

And don't even get me started on what and American and a Briton think constitutes a billion... *sigh*...

-the other Doug

[edstrick]

How many Carl Sagans does it take to screw in a light bulb?

BILLIONS AND BILLIONS!

<you may shoot me now.>

[centsworth_II]

Considering the number of minds Sagan has turned
on to the cosmos, maybe it should be:

How many lightbulbs can Carl Sagan screw in?
Billions and billions.

[CAP-Team]

For those of you who want to "play" with the ephemeris of the extended mission, the official data for the extended tour is now delivered to the NAIF ftp site (for SPICE kernels).

[volcanopele]

hehe:

[volcanopele]

if people can play around with these spice kernel files, the file you are looking for is 070620AP_SCPSE_08102_10191.bsp

[scalbers]

Looks like a double header for Enceladus in 2010 from VP's Celestia images. Should fill in details nicely in leading hemisphere.

[helvick]

hehe:

Hey - where did you get that Titan map??

[volcanopele]

Those graphics are from a few months ago when I was testing out Celestia and spice files. The Titan map I used here is our released Titan map:

http://photojournal.jpl.nasa.gov/catalog/PIA08346

[jasedm]

Does anybody know if the extended mission has been 'ratified' as yet by NASA?

Also, a thought occurs: If the extended mission does go ahead, might there be any advantage in retargeting some instrument pointing for the last few months of the primary mission??

An example: We now know that Cassini is due to come within 500km of Dione in July 2010. This presumably will yield some very good science, and with that in mind, perhaps some observations of Dione between now and July next year (say in the 50,000 - 100,000km range) could be sacrificed in favour of other targets at that time?? i.e. ignore Dione at that time as we know we're getting closer later, and point the instruments elsewhere.

I don't pick on Dione for any particular reason (it could be mid-range shots of Mimas, or Tethys, or the rings) This principle would be especially apposite for instrument pointing at or around periapsis where there are a lot of tempting targets.

Obviously Cassini's trajectory remains the same, and there might be a tiny bonus in terms of usage of the precious hydrazine.

I understand the constraints on time, forward planning, and uploading of information to the spacecraft, but with around a year to affect any changes perhaps the last couple of revolutions could be made 'more useful' in this way?


Jase

[tallbear]

This has multiple answers....

First ... All of the time in the PM has already been planned and is "on the shelf" but small observation tweaks and shifts are being made as they move closer to uplink....but all changes get made in the AfterMarket process which goes on nearly 200 days before Uplink.


Next... Various groups are already busy with science planning for the first revs of the XM. ( In fact most of the high priority science for all of the XM has already been laid out long ago ) The schedule for XM Science Planning is very tight and begins at a royally fast pace begining right around Sept 1 2007.


Not only has the XM been approved but the XM Science planning and Timeline Integration is moving along at high speed

[remcook]

partly related:

http://www.spaceref.com/news/viewpr.html?pid=23130

"Alan Stern, associate administrator for the Science Mission Directorate at NASA Headquarters in Washington, will host a media teleconference at 1 p.m. EDT, Wednesday, Aug. 1. Stern will discuss plans for NASA's Cassini mission to make an unusually close encounter with Saturn's geyser-moon Enceladus and provide project updates for a variety of science flight programs. Planetary and astrophysics program developments also will be discussed"

[jasedm]

Thanks guys - 200 days in advance is quite some forward-planning.
Hat's off once again to the whole Cassini team for this magnificent achievement.
Roll-on the extended-extended mission......

[Harkeppler]

Does anyone have precise information on the Titan surface coverage by SAR during the extended mission?

What other mission proposals had been there (PF6h9 sounds complex)?

Is there any useful visualization of the orbits which can be used to tell the people what is going on?

I am giving planetarium lectures in a larger city and would be interested in these details.

Thanks a lot in advance!

Harkeppler

[pat]

We've moved on from PF6h9 (which was renamed XM0). I think the iterations on this got as far as XM10 before the emergence of the "official delivery of 2-year extended mission trajectory" which is called XM070620. The SPICE kernel of this XM reference trajectory is publically available from the NAIF anonymous FTP site (naif.jpl.nasa.gov:/pub/naif/CASSINI/kernels/spk) and is called 070620AP_SCPSE_08102_10191.bsp for those of you who might be interested in generating your own visualisations.

[TritonAntares]

Hi,
as Solar System Viewer isn't capable of simulating views of the extented mission so far,
is anybody here able to create some views of Iapetus of the following nontargeted encounters with Celestia or another simulation?

2009 07 05 05:00:00 - 2,089,800 km
(low phase, sub-saturn hemisphere, eastern Cassini regio, Snowman craters)

2009 09 11 19:47:00 - 1,161,000 km
(high-phase leading hemisphere, extreme eastern Cassini Regio, western snowman crater)

2009 12 01 15:07:00 - 1,377,900 km
(moderate phase (~100 deg.) , southern leading hemisphere, extreme eastern Cassini Regio, western Snowman crater,western Terra Incognita)

2010 02 21 10:00:00 - 1,469,800 km
(moderate phase (~50 deg.), southern sub-saturn hemisphere, view of eastern Cassini Regio, Snowman craters, Terra Incognita)

2010 05 08 16:30:00 - 1,234,800 km
(high phase (~110 deg.), southern leading hemisphere, crescent view of extreme eastern Cassini Regio and western Terra Incognita)

THX & Bye.

[monitorlizard]

I found a nifty little presentation from the May 2007 Cassini Plasma Spectrometer Team Meeeting, about the extended mission, available at:

caps.space.swri.edu/caps/teamMeetings/teamMeetings.shtml (then click on "Meeting #34", then "extended mission")

The presentation is biased towards particles and fields observations, but still lots of interesting bits of info. A
particularly useful part said "an 'extended, extended' mission should be possible", stating that the prime mission will end with 342.3 m/sec delta-V remaining (95% confidence level) and the PF6H9/XM-9 extended mission is predicted to use 216 m/sec delta-V. However, it went on to say that the reaction wheels may ultimately detemine how much longer Cassini will function (one of four wheels is off due to "bearing cage oscillation", and three wheels are needed for normal operations).

Some cool details on Enceladus encounters I hadn't read before, too. Fifteen pages total, well worth downloading.

[monitorlizard]

Clarification: click on "Minutes from CAPS Team Meeting #34", then "Extended Mission"

[CAP-Team]

Hi,
as Solar System Viewer isn't capable of simulating views of the extented mission so far,
is anybody here able to create some views of Iapetus of the following nontargeted encounters with Celestia or another simulation?

2009 07 05 05:00:00 - 2,089,800 km
(low phase, sub-saturn hemisphere, eastern Cassini regio, Snowman craters)



2009 09 11 19:47:00 - 1,161,000 km
(high-phase leading hemisphere, extreme eastern Cassini Regio, western snowman crater)



2009 12 01 15:07:00 - 1,377,900 km
(moderate phase (~100 deg.) , southern leading hemisphere, extreme eastern Cassini Regio, western Snowman crater,western Terra Incognita)



2010 02 21 10:00:00 - 1,469,800 km
(moderate phase (~50 deg.), southern sub-saturn hemisphere, view of eastern Cassini Regio, Snowman craters, Terra Incognita)



2010 05 08 16:30:00 - 1,234,800 km
(high phase (~110 deg.), southern leading hemisphere, crescent view of extreme eastern Cassini Regio and western Terra Incognita)




THX & Bye.

There you go

[SFJCody]

Cassini's a big spacecraft. Would flying it into the dark material on Iapetus be a good end of mission scenario? Could the impact flash be observed from Earth based telescopes?

[ugordan]

I'm gonna go with: no.
SMART-1 was barely detected and it was in our backyard, Iapetus is waaaaaaay out there...

[SFJCody]

I'm gonna go with: no.
SMART-1 was barely detected and it was in our backyard, Iapetus is waaaaaaay out there...

Yeah, you're right, the numbers don't look at all good for observability. Maybe it would be worth doing to leave a fresh crater for a future spacecraft to the Saturn system to investigate.

[tasp]

A 'Deep Impact' style end of the mission has been raised in another forum, and the suggestion is making it's way up through the appropriate hierarchy.

The goal would be a fresh crater of known age for a future mission to observe, and to perhaps establish a rate for the accumulation of the CR 'crud'.

[Bjorn Jonsson]

A bad idea in my opinion. Iapetus is difficult to reach and the delta-v penalty is significant so if you reach it for a second time you want to observe it, not crash into it. If Cassini has to be destroyed by crashing it into something, crashing it into something easy to reach (and preferably uninteresting) is a better idea I think.

[Gsnorgathon]

Sure would be nice if they could go for a super close-up look at the rings. IIRC Carolyn Porco mentioned something about that way back when (but more along the lines of "sure would be nice" than a plan).

[David]

Obviously, if Cassini were to be placed in an orbit that passed through the rings, the most likely outcome is total destruction of the craft. But if the orbit were perpendicular to the rings (so as to give the shortest travel time), and passed through a known low-density area, isn't there some non-insignificant chance that it might actually survive the passage? Aren't the rings ridiculously narrow, so that if you don't happen to hit a boulder the first time, you might actually be through in a matter of seconds -- maybe sandblasted, but still perhaps in one piece?

[Holder of the Tw...]

Going perpendicular, you'd be through the main part of the rings in a fraction of a second. As to whether there are areas between ringlets that are clear enough of material to give a fighting chance of survival, I'm not sure anyone really knows. Perhaps in the Cassini or Encke divisions. I doubt in the B ring.

I'm not sure from your question whether you are worried or hopeful. One pass survived would certainly send back some very unique data. As to any worry that it might continue to live, don't. If it somehow managed to survive one ring passage, it will just keep coming back on subsequent orbits. Eventually... SPLAT.

[tasp]

I am noting the amazing amount of interest in the current Iapetus encounter, and point out we have not had a Titan or Enceledus encounter generate this kind of interest in a long time.


Therefore:

Be it resolved, the primary goal of the extended, extended mission:

Iapetus

[David]

Going perpendicular, you'd be through the main part of the rings in a fraction of a second. As to whether there are areas between ringlets that are clear enough of material to give a fighting chance of survival, I'm not sure anyone really knows. Perhaps in the Cassini or Encke divisions. I doubt in the B ring.

I'm not sure from your question whether you are worried or hopeful.

I must choose my words more carefully then! It was definitely a hopeful thought. Though I imagine a spacecraft might be better designed than Cassini to maximize the chances of a successful passage.

[mchan]

Paraphrasing from Silent Running: Cassini ain't built for shooting the rapids!

I've always imagined it would be spectacular to go thru the wake of Atlas or Pan.

[tasp]

And I would like to point out, Iapetus still has additional wonders for us to explore!

The 'tiger scratches' (around 30 degrees south, and 355 degrees around) are quite fascinating and I strongly suspect they are also a unique feature we have not encountered (on this scale) elswhere.
Conveniently south of the 'tiger scratches' is also a candidate crater for a large grazing impactor. A possible source for the ring materials.

[rlorenz]

I am noting the amazing amount of interest in the current Iapetus encounter, and point out we have not had a Titan or Enceledus encounter generate this kind of interest in a long time.
Therefore:

Be it resolved, the primary goal of the extended, extended mission:

Iapetus

Dynamically that would be a challenge - because Iapetus is so far out there
the orbits are long, so you get far fewer flybys total.

A lot of the interest in Iapetus is simply because it has only this close flyby -
if there are X interesting things about a body (argue amongst yourselves about
how much X is for Titan vs Europa vs Callisto vs Iapetus etc.) and you
discover some fraction f of the remaining mysteries on the Nth flyby
then
first flyby gives you fX, second gives you (1-f)fX, third gives you (1-f)^2fX findings
etc. After 35 flybys, the incremental value of each indeed goes down. If f is very
small, the rate of novel findings declines only slowly

If the observation/operations are well-understood (as I'd argue is mostly the case
for optical/spectral study of Iapetus etc) then f is large - say 0.3 or something
So this first Iapetus flyby has tons of excitement

Enceladus and Titan had more total surprises and exploited more novel
instrumentation like Radar and INMS which took a while to figure out, and figuring
out how to best acquire VIMS and ISS data through the atmosphere also took some
trial and error) maybe f is smaller, 0.05 or less? (Recall many instruments are
recording seasonal or local time variability, radar sees only 1% of the surface at
a time, etc.)

Sooooo, I agree another flyby or two of Iapetus (particularly to get gravity to understand
its internal structure) would be nice, but the question is how many Titans and Enceladi
to you sacrifice to get them? (And as I've argued above, the answer depends how many
Titans you have under your belt already..) I bet after two or 3 more Iapetus flybys, it
would get pretty uninteresting, whereas Titan's mysteries will endure..

then of course these idealistic arguments have to be modulated by what a dynamically
feasible orbital tour can do (delta-V, flyby geometries, making sure the flyby doesnt happen
in eclipse, or during solar conjunction or something..), and then there is ring science, and
Saturn science, and magnetospheric science to fold in....

p.s. don't underestimate Dione

[ugordan]

p.s. don't underestimate Dione

Do tell! Does it have anything to do with interesting magnetometer readings?

[DrShank]

i agree with ralph's assessment, including the comment about Dione. Dione is probably number 3 in terms of geologic and other complexities, behind Titan and Enchaladas. volcanic smooth plains, extensive graben networks and possible outgassing (all these are being worked on....). it is, or was, a dynamic moon indeed!
paul

[Holder of the Tw...]

Enchaladas?

I've always been in favor of ending the mission with a final Titan series ramping up the orbit so much that Cassini gets catapulted out of the Saturian system. This would preserve the spacecraft as a future museum piece. Either the last couple of orbits or the final escape could be aimed for a last swing past Iapetus.

In my daydreams, I see a final inbound of Cassini passing close by the fully lit trailing hemisphere of Iapetus, with the Snowman in all it's glory, on the way to the last ever Titan pass. Then Cassini is hurled sunward, where the last propellant is used to establish a halo orbit at Saturn's L1, where it continues to observe the planet till the day the reaction wheels give out.